Curable compositions and membranes

ABSTRACT

A curable composition comprising: (i) 2.5 to 50 wt % crosslinker comprising at least two acrylamide groups; (ii) 20 to 65 wt % curable ionic compound comprising an ethylenically unsaturated group and an anionic group; (iii) 15 to 45 wt % solvent; and (iv) 0 to 10 wt % of free radical initiator; wherein the composition has a pH of 0.8 to 12. The compositions are useful for preparing ion exchange membranes.

RELATED APPLICATION DATA

This application is a National Stage Application under 35 U.S.C. 371 ofco-pending PCT application PCT/GB2010/052058 designating the UnitedStates and filed Dec. 9, 2010; which claims the benefit of GB patentapplication number 0921949.4 and filed Dec. 16, 2009 each of which arehereby incorporated by reference in their entireties.

This invention relates to curable compositions, to their use in thepreparation of membranes and to the use of such membranes in ionexchange processes.

Ion exchange membranes are useful in a number of applications, includingelectrodeionisation (EDI), continuous electrodeionisation (CEDI),electrodialysis (ED), electrodialysis reversal (EDR) and capacitivedeionisation used in e.g. flow through capacitors (FTC) for thepurification of water, Donnan or diffusion dialysis (DD) for e.g.fluoride removal or the recovery of acids, pervaporation for dehydrationof organic solvents, fuel cells, electrolysis (EL) of water or forchlor-alkali production, and reverse electrodialysis (RED) whereelectricity is generated from two streams differing in saltconcentration separated by an ion-permeable membrane.

Electrodeionization (EDI) is a water treatment process wherein ions areremoved from aqueous liquids using a membrane and an electricalpotential to effect ion transport. It differs from other waterpurification technologies, such as conventional ion exchange, in that itis does not require the use of chemicals such as acids or caustic soda.EDI can be used to produce ultra pure water.

Electrodialysis (ED) and Electrodialysis Reversal (EDR) areelectrochemical separation processes that remove ions and other chargedspecies from water and other fluids. ED and EDR use small quantities ofelectricity to transport these species through membranes composed of ionexchange material to create separate purified and concentrated streams.Ions are transferred through the membranes by means of direct current(DC) voltage and are removed from the feed fluid as the current drivesthe ions through the membranes to desalinate the process stream. ED andEDR are suitable techniques for producing drinking water. Ion exchangemembranes are also used in Zero Discharge Desalination (ZDD).

A membrane electrode assembly (MEA) appears suitable for a variety ofapplications such as electrolysis, sensors and especially fuel cells.

A flow through capacitor (FTC) is an efficient means of chemical-freeTotal Dissolved Solids (TDS) reduction using electrically charged carbonelectrodes to remove ions.

One of the important problems in the production of ion exchangemembranes is how to provide thin membranes with minimal defects.Desirably the membranes have good permselectivity and low electricalresistance. Additionally the membranes are desired to be strong, whileat the same time being flexible. Flexibility is required for membraneswhich are to be wound into tight circumferential structures. Themembranes also need to retain their physical integrity over an extendedperiod of time. Desirably the method used to prepare the membranes doesnot result in excessive curl. It is also desirable for the membranes tobe resistant to the chemicals that they can come into contact with, e.g.resistant to hydrolysis.

Membrane users require the lowest prices available, which meansproduction processes for the membranes are ideally inexpensive and themembranes should be easily capable of mass production.

US 2008/286627 describes the preparation of membranes for use in fuelcells. Example 6 comprises approximately 10 parts of crosslinker havingtwo acrylamide groups, 40 parts curable ionic monomer and 50 partswater, together with a photoinitiator. However the pH of the compositionused in Example 6 is below 0.1 and the solvent amount is above 45 wt %.

The present invention seeks to provide compositions suitable for use inthe preparation of membranes, in addition to rapid processes forpreparing the membranes, the membranes prepared by the processes andtheir uses.

According to a first aspect of the present invention there is provided acurable composition comprising:

-   (i) 2.5 to 50 wt % crosslinker comprising at least two acrylamide    groups;-   (ii) 20 to 65 wt % curable ionic compound comprising an    ethylenically unsaturated group and an anionic group;-   (iii) 15 to 45 wt % solvent; and-   (iv) 0 to 10 wt % of free radical initiator;    wherein the composition has a pH of 0.8 to 12.

Surprisingly the use of pH 0.8 to 12 results in an improved stability ofthe composition compared to the more acidic compositions used in theexamples of US 2008/286627, US 2005/118479, WO 2009/104470, EP 1987892,JP 2004/146279, JP 2004/335119 and WO 2006/059582. Additionally thepresently claimed pH is less aggressive towards production equipmentsuch as tanks and piping. Furthermore, when the presently claimedcompositions are applied to a porous support to produce a compositemembrane, the milder pH of the composition is less likely to degrade thesupport and good adhesion to the support is more likely.

Preferably the composition has a pH of 1 to 11.

The preferred pH of the composition depends to some extent on whetherthe curable ionic compound is in the free acid or salt form. When thecurable ionic compound is in the free acid form the compositionpreferably has a pH of 1.1 to 5, more preferably 1.1 to 2.5, especiallyabout 1.5. When the curable ionic compound is at least 95% in the saltform the composition preferably has a pH of 2 to 10, more preferably 3to 8, especially 4 to 7 and more especially 4 to 5.

In one embodiment the composition is free from free radical initiatorsor further comprises 0.005 to 10 wt % of photoinitiator.

The crosslinker is preferably present in the composition in an amount of4 to 45 wt %, more preferably 6 to 45 wt %, especially 8 to 40 wt % andmore especially 9 to 25 wt %. A relatively high crosslinker contentgenerally results in a high permselectivity with a high electricalresistance while for a relatively low crosslinker content the formedmembrane structure is more open resulting in a somewhat lowerpermselectivity. A relatively low crosslinker content allows for ahigher content of curable ionic compounds and a higher degree ofswelling, both of which can be useful for obtaining a membrane havinglow electrical resistance.

The ratio of crosslinker:curable ionic compound is selected depending onthe desired properties for the resultant membrane, which in turn dependon the intended use of the membrane.

When a membrane having low electrical resistance is desired, the amountof curable ionic monomer used in the composition is preferably high,while the amount of crosslinker will be reduced in order to accommodatethe higher amount of curable ionic monomer. Thus to prepare membraneshaving low electrical resistance the preferred crosslinker content is 4to 20 wt % (e.g. 4 to 20 wt %), more preferably 6 to 15 wt % (e.g. 6 to15 wt %), especially about 6 to about 12 wt %. With this amount ofcrosslinker, one can still obtain a reasonably strong membrane with goodpermselectivity and without excessive swelling. When a membrane havinghigh permselectivity is desired, the amount of crosslinker present inthe composition will generally be chosen higher, preferably in an amountof 14 to 48 wt %, more preferably from 22 to 43 wt %, especially 28 to38 wt %.

The crosslinker preferably has two or three acrylamide groups, morepreferably two acrylamide groups.

The molecular weight of the crosslinker preferably satisfies theequation:(Y×m)>molecular weight of the crosslinkerwherein m is the number of acrylamide groups in the crosslinker; and

Y is 120, more preferably 105, especially 90, more especially 85 or 77.

The lower values of Y mentioned above are preferred because theresultant crosslinkers crosslink more efficiently than when Y is higher.Furthermore, crosslinkers having the lower values of Y mentioned abovehave lower molecular weights, leaving room for higher amounts of curableionic compound and thereby achieving a lower electrical resistance forthe same level of crosslinking and permselectivity.

The crosslinker is preferably of the Formula (1):

wherein:

-   -   R₁ and R₂ are each independently H or methyl;    -   R₃ and R₄ are each independently H, alkyl, R₃ and R₄ together        with the N groups to which they are attached and Y form an        optionally substituted 6- or 7-membered ring; and    -   Y is an optionally substituted and optionally interrupted        alkylene group.

When R₃ or R₄ is alkyl it is preferably C₁₋₄-alkyl.

When R₃ and R₄ together with the N groups to which they are attached andY form an optionally substituted 6- or 7-membered ring they preferablyform a piperazine, homopiperazine or triazine ring.

The optional interruptions which may be present in Y are preferablyether or, more preferably, amino groups. Y is preferably of the formula—(C_(n)H_(2n))— wherein n is 1, 2 or 3.

As Examples of suitable crosslinkers there may be mentionedN,N′-methylene bis(meth)acrylamide, N,N′-ethylenebis(meth)acrylamide,N,N′-propylenebis(meth)acrylamide, N,N′-butylenebis(meth)acrylamide,N,N′-(1,2-dihydroxyethylene) bis-(meth)acrylamide, 1,4-diacryoylpiperazine, 1,4-bis(acryloyl)homopiperazine,triacryloyl-tris(2-aminoethyl)amine, triacroyl diethylene triamine,tetra acryloyl triethylene tetramine,1,3,5-triacryloylhexahydro-1,3,5-triazine and/or1,3,5-trimethacryloylhexahydro-1,3,5-triazine. The term ‘(meth)’ is anabbreviation meaning that the ‘meth’ is optional, e.g. N,N′-methylenebis(meth) acrylamide is an abbreviation for N,N′-methylene bisacrylamide and N,N′-methylene bis methacrylamide.

More preferably R₃ and R₄ are both H and Y is an optionally substitutedC₁₋₃-alkylene group or an optionally substituted—(C₁₋₃-alkylene-NR₅—C₁₋₃-alkylene)-group wherein R₅ is H or C₁₋₄-alkyl.Especially preferred crosslinkers are N,N′-methylenebis(meth)acrylamide, N,N′-ethylenebis(meth)acrylamide,N,N′-propylenebis(meth)acrylamide, N,N′-(1,2-dihydroxyethylene)bis-(meth)acrylamide, triacryloyl-tris(2-aminoethyl)amine and triacroyldiethylene triamine. The curable ionic compound is preferably present inthe composition in an amount of 25 to 65 wt %, more preferably 35 to 60wt %, especially 40 to 57 wt % and more especially 45 to 55 wt %. Ingeneral, if one wishes to maximise the electrical charge density in themembrane one will choose an amount of curable ionic compound which is ator towards the higher end of the aforementioned ranges.

Preferably the molar ratio of component (i) to (ii) is at least 0.05,more preferably at least 0.10, especially at least 0.15, more especiallyat least 0.2, even more especially at least 0.25. The molar ratio ofcomponent (i) to (ii) may also be at least 0.5, e.g. at least 1. Themolar ratio of component (i) to (ii) is preferably below 1.5, e.g. below1.4, more preferably below 1.0, especially below 0.7, more especiallybelow 0.5.

Preferred curable ionic compounds comprise an acidic group, for examplea sulpho, carboxy and/or phosphato group. In view of the pH of thecomposition these groups will be often be partially or wholly in saltform. The preferred salts are lithium, ammonium, sodium and potassiumsalts and mixtures comprising two or more thereof.

Examples of curable ionic compounds include acrylic acid, beta carboxyethyl acrylate, maleic acid, maleic acid anhydride, vinyl sulphonicacid, phosphonomethylated acrylamide, (2-carboxyethyl)acrylamide and2-(meth)acrylamido-2-methylpropanesulfonic acid.

The total wt % of components (i)+(ii) relative to the total weight ofthe composition is preferably 30 to 90 wt %, more preferably 30 to 85 wt%, especially 40 to 80 wt %, more especially 50 to 75 wt %, particularly58 to 70 wt %, e.g. about 61 wt % or about 65 wt %. If one wishes toavoid swelling of the membrane and lower permselectivity the total wt %of components (i)+(ii) is preferably above 30 wt %.

The curable composition may comprise one or more than one crosslinker ascomponent (i). In a particularly preferred embodiment component (i)consist of crosslinking agent(s) having two acrylamide groups andcomponent (ii) consists of curable ionic compound(s) having oneethylenically unsaturated group and one or more anionic group(s).Preferably the ethylenically unsaturated group in component (ii) is a(meth)acrylamide group because this can result in membranes havingparticularly good resistance to hydrolysis. The most preferred curableionic compound is 2-acrylamido-2-methylpropanesulfonic acid and saltsthereof.

Generally component (i) provides strength to the membrane, whilepotentially reducing flexibility.

When component (ii) has only one acrylamide group (e.g. one H₂C═CHCON<group) it is unable to crosslink. However it is able to react withcomponent (i). Component (ii) can provide the resultant membrane with adesirable degree of flexibility, which is particularly useful inapplications requiring tightly wound membranes. Component (ii) alsoassists the membrane in distinguishing between ions of different chargesby the presence of anionic groups.

In one embodiment the composition comprises less than 10 wt %, morepreferably less than 5 wt %, of ethylenically unsaturated compoundsother than components (i) and (ii). In a preferred embodiment thecomposition is free from ethylenically unsaturated compounds other thancomponents (i) and (ii).

The solvent content of the composition is preferably the minimum, orless than 5% more than the minimum, necessary to achieve the compositionin the form of a homogeneous solution, while at the same time being atleast 15 wt %.

Polar solvents, especially aqueous solvents, are preferred because theseare particularly good at dissolving the curable ionic compound.Preferably at least half, more preferably at least 40 wt %, of thesolvent is water, with the balance comprising organic solvent. Theorganic solvent can be useful for providing a homogenous solution of allthe components of the composition. The inclusion of an organic solventmay also have advantages in the process for preparing the membranebecause many organic solvents will usefully reduce the viscosity and/orsurface tension of the composition, making the manufacturing processeasier in some respects. Preferably the solvent comprises at least 40 wt% water, more preferably at least 60 wt % water. Preferably thecomposition comprises 15 to 45 wt %, more preferably 16 to 40 wt %,especially 20 to 40 wt % and more especially 22 to 35 wt % solvent.

The solvent is preferable water or a mixture comprising water and awater-miscible organic solvent. Due to the presence of a water-miscibleorganic solvent, water-immiscible solvents may also be tolerated insmall amounts such that the overall solvent mixture is miscible.

When the solvent comprises water and an organic solvent the weight ratioof water:organic solvent is preferably higher than 2:3, more preferablybetween 10:1 and 1:1, more preferably between 10:1 and 1:2, especiallybetween 4:1 and 1:1, and more especially between 3:1 and 2:1.

The organic solvent is optionally a single organic solvent or acombination of two or more organic solvents.

Preferred organic solvents include C₁-₄ -alcohols (e.g. methanol,ethanol and propan-2-ol, diols (e.g. ethylene glycol and propyleneglycol), triols (e.g. glycerol), carbonates (e.g. ethylene carbonate,propylene carbonate, dimethyl carbonate, diethyl carbonate, di-t-butyldicarbonate and glycerin carbonate), dimethyl formamide, acetone,N-methyl-2-pyrrolidinone and mixtures comprising two or more thereof. Aparticularly preferred organic solvent is propan-2-ol.

In one embodiment the organic solvent has a low boiling point, e.g. aboiling point below 100° C. Solvents having a low boiling point can beeasily removed by evaporation, avoiding the need for a washing step forremoval of the solvent.

The optimum solvent content for the curable composition depends to someextent on the interaction between the solvent, the curable compound(s)and the crosslinker, and can be determined for each combination bysimple experimentation.

When the composition contains 0% free radical initiator it may be curedusing electron beam radiation.

Preferably the composition comprises 0.01 to 10 wt %, more preferably0.05 to 5 wt %, especially 0.1 to 2 wt % free radical initiator. Thepreferred free radical initiator is a photoinitiator.

The curable composition may comprise one or more than one free radicalinitiator as component (iv).

For acrylamides, diacrylamides, and higher-acrylamides, type Iphotoinitiators are preferred. Examples of type I photoinitiators are asdescribed in WO 2007/018425, page 14, line 23 to page 15, line 26, whichare incorporated herein by reference thereto. Especially preferredphotoinitiators include alpha-hydroxyalkylphenones, e.g.2-hydroxy-2-methyl-1-phenyl propan-1-one and2-hydroxy-2-methyl-1-(4-tert-butyl-) phenylpropan-1-one, andacylphosphine oxides, e.g. 2,4,6-trimethylbenzoyl-diphenylphosphineoxide, and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide.

The curable composition optionally contains (v) 0 to 20 wt %, preferably0 to 10 wt %, of curable compound(s) having one ethylenicallyunsaturated group and no ionic groups.

When a radical initiator is present in the composition, preferably apolymerization inhibitor is also included (e.g. in an amount of below 2wt %). This is useful to prevent premature curing of the compositionduring, for example, storage. Suitable inhibitors include hydroquinone,hydroquinone mono methyl ether, 2,6-di-t-butyl-4-methylphenol,4-t-butyl-catechol, phenothiazine,4-oxo-2,2,6,6-tetramethyl-1-piperidinoloxy, free radical,4-hydroxy-2,2,6,6-tetramethyl-1-piperidinoloxy, free radical,2,6-dinitro-sec-butylphenol, tris(N-nitroso-N-phenylhydroxylamine)aluminum salt, Omnistab™ IN 510 and mixtures comprising two or morethereof.

The curable composition may contain other components, for example acids,pH controllers, preservatives, viscosity modifiers, stabilisers,dispersing agents, antifoam agents, organic/inorganic salts, anionic,cationic, non-ionic and/or amphoteric surfactants, buffers and the like.

A buffer may be used to reduce fluctuations in pH value caused byvariation in concentrations of acidic and basic compounds.

The curable composition may of course contain further components notspecifically mentioned or excluded above.

Curing rates may be increased by including an amine synergist in thecurable composition. Suitable amine synergists are, for example, freealkyl amines, e.g. triethylamine or triethanol amine; aromatic amines,e.g. 2-ethylhexyl-4-dimethylaminobenzoate, ethyl-4-dimethylaminobenzoateand also polymeric amines as polyallylamine and its derivatives.

Curable amine synergists such as ethylenically unsaturated amines (e.g.acrylated amines) are preferable since their use will give less odourdue to their ability to be incorporated into the membrane by curing andalso because they may contain a basic group which can be useful in thefinal (anion permeable) membrane.

When used the amount of amine synergists is preferably from 0.1 to 10wt. % based on the total weight of polymerisable components in thecomposition, more preferably from 0.3 to 3 wt %.

In view of the foregoing a particularly preferred composition comprisesa curable composition comprising:

-   (i) 8 to 16 wt % crosslinker comprising at least two acrylamide    groups;-   (ii) 40 to 60 wt % curable ionic compound comprising an    ethylenically unsaturated group and an anionic group;-   (iii) 22 to 40 wt % solvent; and-   (iv) 0.01 to 2 wt % of photoinitiator;    wherein the composition has a pH of 1 to 11, preferably 1 to 7.

In another embodiment a particularly preferred composition comprises acurable composition comprising:

-   (i) 4 to 30 wt % crosslinker comprising at least two acrylamide    groups;-   (ii) 35 to 60 wt % curable ionic compound comprising an    ethylenically unsaturated group and an anionic group;-   (iii) 26 to 45 wt % solvent; and-   (iv) 0.01 to 2 wt % of photoinitiator;    wherein the composition has a pH of 1 to 11, preferably 1 to 7.

Preferably the ethylenically unsaturated group is a (meth)acrylamidegroup.

Preferably the number of parts of (i), (ii), (iii) and (iv) (whenpresent) in the aforementioned curable compositions add up to 100. Thisdoes not rule out the presence of further, different components, butmerely sets the ratio of the mentioned components relative to eachother.

Preferably the curable composition is free from, or substantially freefrom, methacrylic compounds (e.g. methacrylate and methacrylamidecompounds), i.e. the composition comprises at most 10 wt % of compoundswhich are free from acrylic groups and comprise one or more methacrylicgroups.

Preferably the curable composition is free from, or substantially freefrom, divinyl benzene.

Preferably the curable composition is free from, or substantially freefrom, styrene.

Preferably the curable composition is free from, or substantially freefrom, dyes and pigments. This is because there is no need to includedyes or pigments in the composition.

Thus the preferred curable composition is free from, or substantiallyfree from, divinyl benzene, dyes, pigments, styrene, methacryliccompounds and compounds having tetralkyl-substituted quaternary ammoniumgroups.

According to a second aspect of the present invention there is provideda process for preparing a membrane comprising the following steps:

-   (i) applying a curable composition to a support; and-   (ii) curing the composition to form a membrane;    wherein the curable composition is as defined in the first aspect of    the present invention.

Hitherto such membranes have often been made in slow and energyintensive processes, often having many stages. The present inventionenables the manufacture of membranes in a simple process that may be runcontinuously for long periods of time to mass produce membranesrelatively cheaply.

Optionally the process comprises the further step of separating thecured composition and support. However if desired this further step maybe omitted and thereby a composite membrane is produced comprising thecured composition and a porous support.

The membrane is preferably a cation exchange membrane.

The thickness of the membrane, including the support, when present, ispreferably less than 250 μm, more preferably between 10 and 200 μm, mostpreferably between 20 and 150 μm.

Preferably the membrane has an ion exchange capacity of at least 0.1meq/g, more preferably of at least 0.3 meq/g, especially more than 0.6meq/g, more especially more than 1.0 meq/g, based on the total dryweight of the membrane and any porous support and any porousstrengthening material which remains in contact with the resultantmembrane. Ion exchange capacity may be measured by titration asdescribed by Djugolecki et al, J. of Membrane Science, 319 (2008) onpage 217.

Preferably the membrane has a permselectivity for small cations (e.g.Na⁺) above 90%, more preferably above 95%.

Preferably the membrane has an electrical resistance less than 15ohm·cm², more preferably less than 5 ohm·cm², most preferably less than3 ohm·cm². For certain applications a high electrical resistance may beacceptable especially when the permselectivity is very high, e.g. higherthan 95%. The electrical resistance may be determined by the methoddescribed below in the examples section.

Preferably the membrane exhibits a swelling in water of less than 100%,more preferably less than 75%, most preferably less than 60%. The degreeof swelling can be controlled by the amount of crosslinker, the amountof non-curable compounds and by selecting appropriate parameters in thecuring step and further by the properties of the porous support.

Electrical resistance, permselectivity and % swelling in water may bemeasured by the methods described by Djugolecki et al, J. of MembraneScience, 319 (2008) on pages 217-218.

Typically the ion exchange membrane is substantially non-porous e.g. thepores are smaller than the detection limit of a standard ScanningElectron Microscope (SEM). Thus using a Jeol JSM-6335F Field EmissionSEM (applying an accelerating voltage of 2 kV, working distance 4 mm,aperture 4, sample coated with Pt with a thickness of 1.5 nm,magnification 100,000×, 3° tilted view) the average pore size isgenerally smaller than 5 nm, preferably smaller than 2 nm.

The resultant membrane preferably has a low water permeability so thations may pass through the membrane and water molecules do not pasthrough the membrane. Preferably the membrane's water permeability islower than 1·10⁻⁷ m³/m²·s·kPa, more preferably lower than 1·10⁻⁸m³/m²·s·kPa, most preferably lower than 5·10⁻⁹ m³/m²·s·kPa, especiallylower than 1·10⁻⁹ m³/m²·s·kPa. The requirements for water permeabilitydepend on the intended use of the membrane.

Where desired, a surfactant or combination of surfactants may beincluded in the composition as a wetting agent or to adjust surfacetension. Commercially available surfactants may be utilized, includingradiation-curable surfactants. Surfactants suitable for use in thecomposition include non-ionic surfactants, ionic surfactants, amphotericsurfactants and combinations thereof. Preferably the components of thecurable composition are selected such that no phase separation occursduring the curing step. In this way, the likelihood of a porousstructure in the resultant membrane is reduced.

The network structure of the membrane is determined to a large extent bythe identity of the crosslinking agent(s) and the curable compound andtheir functionality, e.g. the number of crosslinkable groups theycontain per molecule.

During the curing process, the curable composition may form a layer ontop of the support, or it may permeate wholly or partially into thepores of the support thereby forming an impregnated composite membrane.The curable composition may also be applied to both sides of the supportto achieve a symmetrical composite membrane. In a preferred embodimentthe support is saturated with the composition and the saturated supportis cured by EB or UV irradiation.

The process of the present invention may contain further steps ifdesired, for example washing and/or drying the resultant membrane.

Before applying the curable composition to the surface of the support,the support may be subjected to a corona discharge treatment, plasmaglow discharge treatment, flame treatment, ultraviolet light irradiationtreatment, chemical treatment or the like, e.g. for the purpose ofimproving its wettability and the adhesiveness.

The support may also be treated to modify its surface energy, e.g. tovalues above 70 mN/m.

While it is possible to prepare the membrane on a batch basis using astationary support, to gain full advantage of the invention it is muchpreferred to prepare the membrane on a continuous basis using a movingsupport. The support may be in the form of a roll which is unwoundcontinuously or the support may rest on a continuously driven belt (or acombination of these methods). Using such techniques the curablecomposition can be applied to the support on a continuous basis or itcan be applied on a large batch basis.

The curable composition may be applied to the support by any suitablemethod, for example by curtain coating, blade coating, air-knifecoating, knife-over-roll coating, slide coating, nip roll coating,forward roll coating, reverse roll coating, micro-roll coating, dipcoating, foulard coating, kiss coating, rod bar coating or spraycoating. The coating of multiple layers can be done simultaneously orconsecutively. When coating multiple layers the curable compositions maybe the same of different. For simultaneous coating of multiple layers,curtain coating, slide coating and slot die coating are preferred. Thecurable composition(s) may be applied to one side of the support or toboth sides of the support.

In one embodiment at least two of the curable compositions, which may bethe same of different, are applied to the support, e.g. simultaneouslyor consecutively. The curable compositions may be applied to the sameside of the support or to different sides. Thus the application step maybe performed more than once, either with or without curing beingperformed between each application. When applied to different sides theresultant composite membrane may be symmetrical or asymmetrical and thelayers of curable composition may have the same or differentthicknesses. When applied to the same side a composite membrane may beformed comprising at least one top layer and at least one bottom layerthat is closer to the support than the top layer. In this embodiment thetop layer and bottom layer, together with any intervening layers,constitute the membrane and the porous support provides strength to theresultant composite membrane.

Thus in a preferred process, the curable composition is appliedcontinuously to a moving support, more preferably by means of amanufacturing unit comprising one or more curable compositionapplication station(s), one or more irradiation source(s) for curing thecomposition, a membrane collecting station and a means for moving thesupport from the curable composition application station(s) to theirradiation source(s) and to the membrane collecting station.

The curable composition application station(s) may be located at anupstream position relative to the irradiation source(s) and theirradiation source(s) is/are located at an upstream position relative tothe membrane collecting station.

In order to produce a sufficiently flowable curable composition forapplication by a high speed coating machine, it is preferred that thecurable composition has a viscosity below 5000 mPa·s when measured at35° C., more preferably from 1 to 1500 mPa·s when measured at 35° C.Most preferably the viscosity of the curable composition is from 2 to500 mPa·s when measured at 35° C. For coating methods such as slide beadcoating the preferred viscosity is from 2 to 150 mPa·s when measured at35° C.

With suitable coating techniques, the curable composition may be appliedto a support moving at a speed of over 5 m/min, preferably over 10m/min, more preferably over 15 m/min, e.g. more than 20 m/min, or evenhigher speeds, such as 60 m/min, 120 m/min or up to 400 m/min can bereached.

Curing is preferably performed by radical polymerisation, preferablyusing electromagnetic radiation. The source of radiation may be anysource which provides the wavelength and intensity of radiationnecessary to cure the composition. A typical example of a UV lightsource for curing is an D-bulb with an output of 600 Watts/inch (240W/cm) as supplied by Fusion UV Systems. Alternatives are the V-bulb andthe H-bulb from the same supplier.

When no photo-initiator is included in the curable composition, thecomposition can be cured by electron-beam exposure, e.g. using anexposure of 50 to 300 keV. Curing can also be achieved by plasma orcorona exposure

During curing the components (i) and (ii) polymerise to form a polymericmembrane. The curing may be brought about by any suitable means, e.g. byirradiation and/or heating. Preferably curing occurs sufficientlyrapidly to form a membrane within 30 seconds. If desired further curingmay be applied subsequently to finish off, although generally this isnot necessary.

The curing is preferably achieved thermally (e.g. by irradiating withinfrared light) or, more preferably, by irradiating the composition withultraviolet light or an electron beam.

For thermal curing the curable composition preferably comprises one ormore thermally reactive free radical initiators, preferably beingpresent in an amount of 0.01 to 5 parts per 100 parts of curablecomposition, wherein all parts are by weight.

Examples of thermally reactive free radical initiators include organicperoxides, e.g. ethyl peroxide and/or benzyl peroxide; hydroperoxides,e.g. methyl hydroperoxide, acyloins, e.g. benzoin; certain azocompounds, e.g. α,α′-azobisisobutyronitrile and/orγ,γ′-azobis(γ-cyanovaleric acid); persulfates; peracetates, e.g. methylperacetate and/or tert-butyl peracetate; peroxalates, e.g. dimethylperoxalate and/or di(tert-butyl) peroxalate; disulfides, e.g. dimethylthiuram disulfide and ketone peroxides, e.g. methyl ethyl ketoneperoxide. Temperatures in the range of from about 30° C. to about 150°C. are generally employed for infrared curing. More often, temperaturesin the range of from about 40° C. to about 110° C. are used.

Preferably curing of the curable composition begins within 3 minutes,more preferably within 60 seconds, after the composition has beenapplied to the support.

Preferably the curing is achieved by irradiating the curable compositionfor less than 30 seconds, more preferably less than 10 seconds,especially less than 3 seconds, more especially less than 2 seconds. Ina continuous process the irradiation occurs continuously and the speedat which the curable composition moves through the beam of irradiationis mainly what determines the time period of curing.

Preferably the curing uses ultraviolet light. Suitable wavelengths arefor instance UV-A (390 to 320 nm), UV-B (320 to 280 nm), UV-C (280 to200 nm) and UV-V (445 to 395 nm), provided the wavelength matches withthe absorbing wavelength of any photo-initiator included in the curablecomposition.

Suitable sources of ultraviolet light are mercury arc lamps, carbon arclamps, low pressure mercury lamps, medium pressure mercury lamps, highpressure mercury lamps, swirlflow plasma arc lamps, metal halide lamps,xenon lamps, tungsten lamps, halogen lamps, lasers and ultraviolet lightemitting diodes. Particularly preferred are ultraviolet light emittinglamps of the medium or high pressure mercury vapour type. In most caseslamps with emission maxima between 200 and 450 nm are particularlysuitable.

The energy output of the irradiation source is preferably from 20 to1000 W/cm, preferably from 40 to 500 W/cm but may be higher or lower aslong as the desired exposure dose can be realized. The exposureintensity is one of the parameters that can be used to control theextent of curing which influences the final structure of the membrane.Preferably the exposure dose is at least 40 mJ/cm², more preferablybetween 40 and 1500 mJ/cm², most preferably between 70 and 900 mJ/cm² asmeasured by an High Energy UV Radiometer (UV PowerMap™ from EIT, Inc) inthe UV-A and UV-B range indicated by the apparatus. Exposure times canbe chosen freely but preferably are short and are typically less than 10seconds, more preferably less than 5 seconds, especially less than 3seconds, more especially less than 2 seconds, e.g. between 0.1 and 1second.

To reach the desired exposure dose at high coating speeds, more than oneUV lamp may be used, so that the curable composition is irradiated morethan once. When two or more lamps are used, all lamps may give an equaldose or each lamp may have an individual setting. For instance the firstlamp may give a higher dose than the second and following lamps or theexposure intensity of the first lamp may be lower. Varying the exposuredose of each lamp may influence the polymer matrix structure and thefinal crosslink density. In a preferred embodiment the composition iscured by simultaneous irradiation from opposite sides using two or moreirradiation sources, e.g. two lamps (one at each side). The two or moreirradiation sources preferably irradiate the composition with the sameintensity as each other. By using this symmetric configuration, a highercrosslinking efficiency can be achieved and curling of the membrane canbe reduced or prevented.

Photoinitiators may be included in the curable composition, as mentionedabove, and are usually required when curing uses UV or visible lightradiation. Suitable photoinitiators are those known in the art.

Curing by irradiation with UV or electron beam is preferably performedat between 20 and 60° C. While higher temperatures may be used, theseare not preferred because they can lead to higher manufacturing costs.

Preferred supports are porous, e.g. they may be a woven or non-wovensynthetic fabric, e.g. polyethylene, polypropylene, polyacrylonitrile,polyvinyl chloride, polyester, polyamide, and copolymers thereof, orporous membranes based on e.g. polysulfone, polyethersulfone,polyphenylenesulfone, polyphenylenesulfide, polyimide, polyethermide,polyamide, polyamideimide, polyacrylonitrile, polycarbonate,polyacrylate, cellulose acetate, polypropylene, poly(4-methyl1-pentene), polyinylidene fluoride, polytetrafluoroethylene,polyhexafluoropropylene, polychlorotrifluoroethylene, and copolymersthereof.

Various porous supports are available commercially, e.g. fromFreudenberg Filtration Technologies (Novatexx materials) and Sefar AG.

Surprisingly, ion exchange membranes with anionic groups (e.g. sulpho,carboxyl and phosphato groups) can exhibit good properties in terms oftheir permselectivity and conductivity while at the same time being notoverly expensive to manufacture by the present process.

The present process allows the preparation of membranes having adesirable degree of flexibility, without being overly flexible or toorigid. The presence of the solvent improves coatability for the curablecomposition and can provide thin membranes with low numbers of defects,low tendency to curl while retaining good durability in use.

According to a third aspect of the present invention there is provided amembrane obtained by a process according to the second aspect of thepresent invention.

The membranes according to the third aspect of the present invention mayalso be put to other uses requiring membranes having anionic groups.

The membranes according to the third aspect of the present inventionpreferably have the properties described above in relation to the secondaspect of the present invention.

The membranes of the invention are particularly useful for ED, (C)EDI,EDR, FTC, ZDD and RED, although they may also be used for otherpurposes.

According to a fourth aspect of the present invention there is provideduse of a membrane according to the third aspect of the present inventionfor water purification or for the generation of electricity.

According to a fourth aspect of the present invention there is providedan electrodialysis or reverse electrodialysis unit, anelectrodeionization module or a flow through capacitor comprising one ormore membranes according to the third aspect of the present invention.The electrodeionization module is preferably a continuouselectrodeionization module.

Preferably the electrodialysis or reverse electrodialysis unit or theelectrodeionization module or the flow through capacitor comprises atleast one anode, at least one cathode and one or more membrane accordingto the third aspect of the present invention. Further the unitpreferably comprises an inlet for providing a flow of relatively saltywater along a first side of a membrane according to the presentinvention and an inlet for providing a less salty flow water along asecond side of the membrane such that ions pass from the first side tothe second side of the membrane. Preferably the one or more membranes ofthe unit comprise a membrane according to the third aspect of thepresent invention having anionic groups and a further membrane havingcationic groups.

In a preferred embodiment the unit comprises at least 3, more preferablyat least 5, e.g. 36, 64 or up to 500, membranes according to the thirdaspect of the present invention, the number of membranes being dependenton the application. The membrane may for instance be used in aplate-and-frame or stacked-disk configuration or in a spiral-wounddesign. Alternatively, a continuous first membrane according to thepresent invention having anionic groups may be folded in a concertina(or zigzag) manner and a second membrane having cationic groups (i.e. ofopposite charge to the first membrane) may be inserted between the foldsto form a plurality of channels along which fluid may pass and havingalternate anionic and cationic membranes as side walls.

The invention will now be illustrated with non-limiting examples whereall parts and percentages are by weight unless specified otherwise.

In the examples the following properties were measured by the methodsdescribed below.

General Test Methods

Permselectivity was measured by using a static membrane potentialmeasurement. Two cells are separated by the membrane underinvestigation. Prior to the measurement the membrane was equilibrated ina 0.1 M NaCl solution for at least 12 hours. Two streams havingdifferent NaCl concentrations were passed through cells on oppositesides of the membranes under investigation. One stream had aconcentration of 0.1M NaCl (from Sigma Aldrich, min. 99.5% purity) andthe other stream was 0.5 M NaCl. The flow rate of both streams was 0.90dm³/min. Two Calomel reference electrodes (from Metrohm AG, Switzerland)were connected to Haber-Luggin capillary tubes that were inserted ineach cell and were used to measure the potential difference over themembrane. The effective membrane area was 3.14 cm² and the temperaturewas 21° C.

When a steady state was reached, the membrane potential was measured(ΔV_(meas)).

The permselectivity (α(%)) of the membrane was calculated according theformula:α (%)=ΔV _(meas) /ΔV _(theor)*100%.

The theoretical membrane potential (ΔV_(theor)) is the potential for a100% permselective membrane as calculated using the Nernst equation.

To compensate for day-to-day measurement fluctuations in all a (%)measurements an internal standard was included which was used tonormalize the results. The internal standard used was CMX membrane fromTokuyama Soda; its α (%) value was determined to be 98%.

Electrical resistance ER (ohm·cm²) was measured by the method describedby Djugolecki et al, J. of Membrane Science, 319 (2008) on page 217-218with the following modifications:

-   -   the auxiliary membranes were CMX and AMX from Tokuyama Soda,        Japan;    -   a Cole Parmer masterflex console drive (77521-47) with easy load        II model 77200-62 gear pumps was used for all compartments;    -   the flowrate of each stream was 475 ml/min controlled by Porter        Instrument flowmeters (type 150AV-B250-4RVS) and Cole Parmer        flowmeters (type G-30217-90);    -   the effective area of the membrane was 3.14 cm².

The burst strength of the membrane was determined by measuring the burststrength according ISO 2758, using a Burst tester EC35 from TestingMachines Inc., using the following settings:

-   -   Pump flow 95 ml/min;    -   Sensitivity 95% Pmax;    -   Threshold 70;    -   Burst deceleration 50 ms;    -   Stop deceleration 20 ms;    -   Weight 1 gr/m²;    -   Coversheet 0 gr/m²; and    -   Nr of samples is 1.

The pH of the compositions was measured using a Metrohm 691 pH meterequipped with a Metrohm 6.0228.000 electrode, calibrated at 20° C. withstandard buffers of pH 4 and 7.

Ingredients

MBA is N,N′-methylene bisacrylamide from Sigma Aldrich.

EBA is N,N′-ethylene bisacrylamide from Sigma Aldrich.

BAHP is 1,4-bis(acryloyl) homopiperazine, synthesized as described in WO2010/106356

AMPS is 2-Acryloylamido-2-methylpropanesulfonic acid from Hang-Zhou(China).

HDMAP is 2-hydroxy-2-methyl-1-phenyl-propan-1-one, a photoinitiator fromCytec.

LiOH.H₂O is lithium hydroxide monohydrate from Chemetall.

MeHQ is hydroquinone monomethyl ether, a polymerisation inhibitor fromMerck.

IPA is 2-propanol from Shell.

Viledon® Novatexx 2597 is a nonwoven polyamide material from FreudenbergFiltration Technologies.

NaOH, KOH, 30% ammonia, glycine and glycerol are commodity chemicals andwere used without further purification.

EXAMPLES 1 TO 23 AND COMPARATIVE EXAMPLES 1 TO 6

Curable compositions CC1 to CC23 and comparative curable compositionsCE1 to CE6 were prepared by mixing at a temperature of 65° C. theingredients expressed as wt % shown in Tables 1 to 55.

The resultant curable compositions (described in Tables 1 to 5) wereapplied to an aluminium underground carrier using a 150 μm wire woundbar, at a speed of approximately 5 m/min, by hand, followed byapplication to a non-woven support (Viledon® Novatexx™ 2597) levelledusing a 4 micrometer wire wound rod coater. The temperature of thecurable compositions was about 50° C. during coating and somewhat lowerjust before curing.

A membrane was prepared by curing the coated support using a LightHammer LH6 from Fusion UV Systems fitted with a D-bulb working at 100%intensity with a speed of 30 m/min (single pass). The exposure time was0.47 seconds.

After curing, the membrane was stored in a 0.1 M NaCl solution for atleast 12 hours. Membranes of the invention obtained from compositionshaving a pH of 0.8 to 12 did not need a washing step in pH 6 buffer.Instead they were washed with an NaCl solution. Membranes made from AMPSwithout neutralization were washed with a pH 6 buffer.

In the calculation of the solvent content the solvents present as partof an ingredient is included and crystal water of ingredients is treatedas solvent.

TABLE 1 Curable Composition Ingredient CC1 CC2 CC3 CC4 CC5 CE1 CE2 CE3CE4 AMPS 46.83 46.57 49.17 52.46 47.27 51.72 49.06 34.24 39.78 MBA 11.1611.09 11.72 0 12.31 11.68 17.81 9.95 EBA 0 0 0 13.64 0 0 0 0 0 BAHP 0 00 0 15.23 0 0 0 0 HDMAP 1.00 0.95 1.00 1.08 0.5 1.00 0.97 0.01 0.05Water + 1000 22.65 22.52 20.25 16.00 19.83 24.98 23.73 34.24 50.22 ppmMeHQ IPA 9.06 9.01 8.10 6.40 7.79 9.99 9.49 13.7 0 LiOH•H₂O 9.30 9.869.76 10.42 9.38 0 5.07 0 0 Total 100 100 100 100 100 100 100 100 100 pH1.31 10.63 1.35 4.61 1.77 −0.10 0.31 0.05 0.08 α (%) 95.3 93.0 94.4 95.594.4 96.8 95.7 — — ER (ohm · cm²) 4.2 2.1 2.9 2.4 2.4 1.9 2.5 — — Burststrength 735 680 233 — — (kPa)

The curable compositions CE3 and CE4 were prepared according thedescription in JP2004146279 and US20080286627 respectively.

TABLE 2 Curable Composition Ingredient CC6 CC7 CC8 AMPS 49.30 50.0449.28 MBA 11.75 11.92 11.74 HDMAP 0.5 1.00 0.5 Water + 1000 ppm 20.4916.79 17.15 MeHQ IPA 8.12 6.72 8.11 NaOH 9.34 0 0 KOH 0 13.53 0 NH₄OH(30%) 0 0 13.22 Total 100 100 100 pH* 0.81 1.15 0.84 α (%) 92.3 90.392.8 ER (ohm · cm2) 2.4 1.6 2.6 Burst strength (kPa) 648 680 581 *The pHwas measured at a temperature of 40° C. using an Orion A290 pH-meter,calibrated at 40° C. with standard buffers of pH 4 and 7.

TABLE 3 Curable Composition Ingredient CE5 CE6 CC9 CC10 CC11 CC12 CC13AMPS 34.64 40.06 52.21 46.83 51.74 51.79 28.75 MBA 8.25 9.54 5.18 11.1612.33 12.34 0 BAHP 0 0 0 0 0 0 43.33 HDMAP 0.50 0.77 0.91 1.00 0.79 1.000.50 Water + 1000 44.03 29.76 22.38 22.65 17.76 17.38 12.25 ppm MeHQ IPA5.71 11.91 8.95 9.06 7.10 0 9.46 glycerol 0 0 0 0 0 6.95 0 LiOH•H₂O 6.887.96 10.37 9.30 10.28 10.53 5.71 pH 1.08 1.08 1.13 1.20 1.23 1.15 1.91Solvent content 55.5 48.3 40.0 39.5 33.5 33.1 26 (wt %) α (%) 84.2 87.991.8 95.3 94.4 94.6 97.2 ER (ohm · cm²) 1.7 2.7 3.5 4.2 3.3 3.3 14.9

TABLE 4 Curable Composition Ingredient CC14 CC15 CC16 CC17 CC18 CC19AMPS 46.83 46.47 46.05 47.12 47.30 47.33 MBA 11.16 11.07 10.96 11.2211.26 11.27 HDMAP 1.00 1.82 2.72 0.45 0.09 0.01 Water + 1000 22.65 22.4322.23 22.75 22.83 22.85 ppm MeHQ IPA 9.06 8.98 8.89 9.10 9.13 9.14LiOH•H₂O 9.30 9.23 9.15 9.36 9.39 9.40 pH 1.18 1.11 1.32 1.18 1.21 1.06α (%) 95.3 96.0 95.4 95.7 94.7 95.2 ER (ohm · cm²) 4.2 4.1 3.3 4.1 3.74.0

TABLE 5 Curable Composition Ingredient CC20 CC21 CC22 CC23 AMPS 48.7148.22 48.55 48.07 MBA 11.60 11.49 11.57 11.46 HDMAP 0.50 0.50 0.50 0.49Water + 1000 ppm 20.41 20.21 20.35 20.15 MeHQ IPA 8.02 7.94 8.00 7.92LiOH•H₂O 9.76 9.66 10.03 9.93 glycine 0 0.99 0 0.99 surfactant 1.0 0.991.0 0.99 pH 0.87 2.61 9.39 7.27 α (%) 93.7 93.6 92.5 90.3 ER (ohm · cm2)2.2 2.3 2.0 2.0

The invention claimed is:
 1. A curable composition comprising: (i) 2.5to 50 wt % crosslinker comprising at least two acrylamide groups; (ii)20 to 65 wt % curable ionic compound comprising an ethylenicallyunsaturated group and an anionic group; (iii) 15 to 45 wt % solvent; and(iv) 0 to 10 wt % of free radical initiator; wherein the composition hasa pH of 0.8 to
 12. 2. A composition according to claim 1 wherein thetotal wt % of components (i)+(ii) relative to the total weight of thecomposition is 30 to 85 wt %.
 3. A composition according to claim 1which comprises 20 to 40 wt % solvent.
 4. A composition according toclaim 1 wherein the molar ratio of component (i) to (ii) is at least0.05 and below 1.0.
 5. A composition according to claim 1 comprising 20to 40 wt % solvent and wherein the molar ratio of component (i) to (ii)is at least 0.05 and below 1.0.
 6. A composition according to claim 5wherein the solvent comprises water and a water-miscible organicsolvent.
 7. A composition according to claim 1 which comprises 0.005 to10 wt % photoinitiator.
 8. A composition according to claim 1 whereinthe ethylenically unsaturated group is an acrylamide or a methacrylamidegroup.
 9. A composition according to claim 1 comprising: (i) 8 to 16wt %crosslinker comprising at least two acrylamide groups; (ii) 40 to 60 wt% curable ionic compound comprising an ethylenically unsaturated groupand an anionic group; (iii) 22 to 40 wt % solvent; and (iv) 0.01 to 2 wt% of photoinitiator; wherein the composition has a pH of 1 to
 7. 10. Aprocess for preparing a membrane comprising the following steps: (i)applying a curable composition to a support; and (ii) curing thecomposition to form a membrane; wherein the curable composition is asdefined in claim
 1. 11. A process according to claim 10 wherein thecomposition is cured by irradiation with an electron beam or UV lightfor a period of less than 30 seconds.
 12. A process according to claim10 wherein the curable composition is applied continuously to a movingsupport using a manufacturing unit comprising a curable compositionapplication station, an irradiation source for curing the composition,and a membrane collecting station wherein the support moves from thecurable composition application station to the irradiation source and tothe membrane collecting station.
 13. A composition according to claim 1comprising 20 to 40 wt % solvent and wherein the ethylenicallyunsaturated group is an acrylamide or a methacrylamide group.
 14. Acomposition according to claim 9 wherein the molar ratio of component(i) to (ii) is at least 0.05 and below 1.0.
 15. A composition accordingto claim 9 wherein the ethylenically unsaturated group is an acrylamideor a methacrylamide group.